Full flow / Dual staged expander cycle

[Ke8ort]

Don't know if this will help answer your question, but here you go:

The purpose of preburners and heat exchangers in the context of a rocket engine is to add more energy to the fuel flow, almost always to drive a turbine which in turns takes some of that energy and uses it to drive a pump to move more fuel through the system. That's what makes it a "cycle".

In an expander cycle engine, energy is added to the fuel via the cooling channels surrounding the nozzle/combustion chamber. The nozzle is acting as a heat exchanger, exchanging heat from the rocket exhaust into the fuel flow; in other words, adding energy to the fuel while taking it away from the exhaust. The thermal energy of the exhaust is transferred into the fuel flow, and then converted into mechanical energy via a turbine. This is a "two birds with one stone" solution since it adds energy to our fuel flow, allowing us to drive our pump, while also keeping the nozzle cool. The expense of an expander cycle engine is that you are taking some of the thermal energy out of the exhaust.

In a staged-combustion cycle engine, a small portion of the fuel and oxidizer are burned in order to release their chemical energy and drive a turbine. This turbine takes that released chemical energy and turns it into mechanical energy which drives a pump to pump more fuel/oxidizer into the engine. There's your cycle again. We can also take advantage of regenerative cooling, just like in an expander cycle engine because we don't want to melt our nozzle. The cost of the staged combustion cycle is that we are taking some of our fuel that could be burned in the combustion chamber, and instead using it to drive a turbine to pump more fuel into the system.

Now, with a full flow expander cycle engine, you run into the challenge that all of your "pumping energy" needs to come from your heat exchanger. When trying to get enough energy to meaningfully drive fuel/oxidizer, your heat exchanger is now the bottleneck, and you aren't able to harvest enough energy due to limitations in the size/geometry of the engine. You'll end up with a "chicken and the egg" scenario where to drive the pumps, you need more energy from the combustion chamber, but to get more combustion energy, you need to pump more.

With staged combustion, you can always burn more fuel/oxidizer if you need more pumping power since the "pumping energy" is generated independent from the main combustion process of the engine.

[InterestedEngineer]

Don't know if this will help answer your question, but here you go:

The purpose of preburners and heat exchangers in the context of a rocket engine is to add more energy to the fuel flow, almost always to drive a turbine which in turns takes some of that energy and uses it to drive a pump to move more fuel through the system. That's what makes it a "cycle".

In an expander cycle engine, energy is added to the fuel via the cooling channels surrounding the nozzle/combustion chamber. The nozzle is acting as a heat exchanger, exchanging heat from the rocket exhaust into the fuel flow; in other words, adding energy to the fuel while taking it away from the exhaust. The thermal energy of the exhaust is transferred into the fuel flow, and then converted into mechanical energy via a turbine. This is a "two birds with one stone" solution since it adds energy to our fuel flow, allowing us to drive our pump, while also keeping the nozzle cool. The expense of an expander cycle engine is that you are taking some of the thermal energy out of the exhaust.

In a staged-combustion cycle engine, a small portion of the fuel and oxidizer are burned in order to release their chemical energy and drive a turbine. This turbine takes that released chemical energy and turns it into mechanical energy which drives a pump to pump more fuel/oxidizer into the engine. There's your cycle again. We can also take advantage of regenerative cooling, just like in an expander cycle engine because we don't want to melt our nozzle. The cost of the staged combustion cycle is that we are taking some of our fuel that could be burned in the combustion chamber, and instead using it to drive a turbine to pump more fuel into the system.

Now, with a full flow expander cycle engine, you run into the challenge that all of your "pumping energy" needs to come from your heat exchanger. When trying to get enough energy to meaningfully drive fuel/oxidizer, your heat exchanger is now the bottleneck, and you aren't able to harvest enough energy due to limitations in the size/geometry of the engine. You'll end up with a "chicken and the egg" scenario where to drive the pumps, you need more energy from the combustion chamber, but to get more combustion energy, you need to pump more.

With staged combustion, you can always burn more fuel/oxidizer if you need more pumping power since the "pumping energy" is generated independent from the main combustion process of the engine.

Just to add the obvious that some still miss:  liquids and gases flow from the place of high pressure to the place of low pressure.  So much of the "energy added" by the preburner or expander is use to pressurize the fuel/oxidizer with a pump such that when combusted, the exhaust doesn't go backwards, it goes downhill towards the exhaust nozzle.  You can see this in the Raptor wikipedia page where the pumps pressurize the methalox to about 30-50 bar more than the pressure in the combustion chamber

Since an expander cycle can't put as much energy (in the form of pressure) into the fuel before it enters the combustion chamber, the combustion chamber operates at a much lower pressure in such a system as compared to a staged combustion system.  Per the Da Lavel equation lower pressure means lower exhaust velocity (and lower thrust) per unit mass of propellant.

[russianhalo117]

And to complicate things advanced expander cycle engines that have recently been researched and developed as listed in NTRS etal can expand capabilities by adding one or more additional heat exchangers into the circuit.

[Skye]

And to complicate things advanced expander cycle engines that have recently been researched and developed as listed in NTRS etal can expand capabilities by adding one or more additional heat exchangers into the circuit.

Wait what?

Don't know if this will help answer your question, but here you go:

The purpose of preburners and heat exchangers in the context of a rocket engine is to add more energy to the fuel flow, almost always to drive a turbine which in turns takes some of that energy and uses it to drive a pump to move more fuel through the system. That's what makes it a "cycle".

In an expander cycle engine, energy is added to the fuel via the cooling channels surrounding the nozzle/combustion chamber. The nozzle is acting as a heat exchanger, exchanging heat from the rocket exhaust into the fuel flow; in other words, adding energy to the fuel while taking it away from the exhaust. The thermal energy of the exhaust is transferred into the fuel flow, and then converted into mechanical energy via a turbine. This is a "two birds with one stone" solution since it adds energy to our fuel flow, allowing us to drive our pump, while also keeping the nozzle cool. The expense of an expander cycle engine is that you are taking some of the thermal energy out of the exhaust.

In a staged-combustion cycle engine, a small portion of the fuel and oxidizer are burned in order to release their chemical energy and drive a turbine. This turbine takes that released chemical energy and turns it into mechanical energy which drives a pump to pump more fuel/oxidizer into the engine. There's your cycle again. We can also take advantage of regenerative cooling, just like in an expander cycle engine because we don't want to melt our nozzle. The cost of the staged combustion cycle is that we are taking some of our fuel that could be burned in the combustion chamber, and instead using it to drive a turbine to pump more fuel into the system.

Now, with a full flow expander cycle engine, you run into the challenge that all of your "pumping energy" needs to come from your heat exchanger. When trying to get enough energy to meaningfully drive fuel/oxidizer, your heat exchanger is now the bottleneck, and you aren't able to harvest enough energy due to limitations in the size/geometry of the engine. You'll end up with a "chicken and the egg" scenario where to drive the pumps, you need more energy from the combustion chamber, but to get more combustion energy, you need to pump more.

With staged combustion, you can always burn more fuel/oxidizer if you need more pumping power since the "pumping energy" is generated independent from the main combustion process of the engine.

Oh, ok. I think I get it now? 

[Ke8ort]

Oh, ok. I think I get it now? 

I guess to answer your very first question, "could you design a full-flow/dual expander cycle engine?", technically yes, but probably not a good one. In engineering, every design has it's tradeoffs, some more than others. At first glance, a dual expander cycle engine could be great and offer the efficiency of both expander and full-flow cycles, however, realistically, the tradeoffs needed to make this happen are too large.

In order for this to work, you'd need to maximize the surface area of the combustion chamber and nozzle in order to reduce the bottleneck of your heat exchanger. This probably mean having a very long and skinny combustion chamber and a very large nozzle. Then there's the issue that since your bottlenecked in the amount of energy that can be transferred from the combustion process into the fuel, your pressure to drive the pump and inject into the engine is lower. This will results in lower combustion pressures and slower exhaust velocities, further impacting the performance of the engine.

For an engineer, these design difficulties and the tradeoffs that come with them are probably a non-starter. Staged combustion may be more complex, but when it comes to designing a reliable, performant, efficient engine that works across a wide variety of conditions and offers good TWR, other cycles offer more room to play with and optimize than in a heavily constrained problem like that of a dual-expander cycle engine.

[Skye]

Ah, I see. Thank you! 

[InterestedEngineer]

The raptor is technically dual staged  full flow with single-sided (CH4) expander cycle because LCH4 cooling is needed for the throat, chamber, and nozzle.  That energy doesn't go to waste, I suspect the CH4 pump uses slightly less power than the simple calculation case because of the heat transferred to the coolant loop.

But flowing oxygen to cool things?  Yikes, the materials for just the oxygen preburner and pumps are already exotic enough, you have to severely limit the surface area of that to keep reliability and costs to a reasonable level.   You'll note on the Raptor diagram the smallest surface area is the LOX preburner/pump side.  It's straight in to the combustion chamber ASAP.

Oxygen really wants to oxidize, that's what it does, and it'll do it to your metal cooling channels quite happily.

[Gliderflyer]

The raptor is technically dual staged  full flow with single-sided (CH4) expander cycle because LCH4 cooling is needed for the throat, chamber, and nozzle.  That energy doesn't go to waste, I suspect the CH4 pump uses slightly less power than the simple calculation case because of the heat transferred to the coolant loop.

But flowing oxygen to cool things?  Yikes, the materials for just the oxygen preburner and pumps are already exotic enough, you have to severely limit the surface area of that to keep reliability and costs to a reasonable level.   You'll note on the Raptor diagram the smallest surface area is the LOX preburner/pump side.  It's straight in to the combustion chamber ASAP.

Oxygen really wants to oxidize, that's what it does, and it'll do it to your metal cooling channels quite happily.

I've worked on a dual expander engine, and I would much rather do that than an ox-rich preburner. Oxygen cooling is a lot easier than most people think (you can do some cursed stuff and it doesn't care). Dynetics, Rotary Rocket, XCOR, Launcher, Blue Origin, Masten, and a bunch of others I'm probably forgetting have done LOX cooling.

[Skye]

So LOX cooling is almost as efficient as a preburner?

[edzieba]

So LOX cooling is almost as efficient as a preburner?

No.

[Skye]

Oh. It’s just simpler and more appealing to work with than a preburner because it’s easier to handle?

[edzieba]

Oh. It’s just simpler and more appealing to work with than a preburner because it’s easier to handle?

You're trying to comapre two different things. "LOX cooling" is a design decision a "preburner" is a specific component. It'd be like trying to compare a car alternator vs. 4-wheel drive - they're not comparable.

[Skye]

I see what you mean, but they both heat the LOX - no?